Rapid advances in digital technology have produced corresponding advances in digital image signal processing in various fields such as high definition television (HDTV). The MPEG (Motion Picture Experts Group) signal compression standard for MPEG-2 video processing (ISO/IEC International Standard 13818-2, Jan. 20, 1995) is a related development. This widely accepted image processing standard has been found to be particularly attractive for use with satellite, cable and terrestrial broadcast systems including HDTV systems.
A digital HDTV terrestrial broadcast system recently adopted as the Grand Alliance HDTV system in the United States defines a standard of digital broadcast of high definition (HD) program material which has been data compressed using the MPEG-2 compression standard. A description of the Grand Alliance HDTV system is found, for example, in the 1994 Proceedings of the National Association of Broadcasters, 48th Annual Broadcast Engineering Conference Proceedings, Mar. 20–24, 1994. The HD broadcast standard provides for image resolution up to 1920 pixels per line (horizontally) by 1080 lines (vertically). The MPEG-2 standard defines the procedures required to decompress the HD image for reproduction by a display device such as in a television receiver. About 80 Mega bits (Mb) of memory is required by an MPEG decoder to properly decode an HD image as defined in the terrestrial broadcast standard. About 96 Mb of memory would be required in a consumer receiver.
In an MPEG video signal decoder such as may be found in a television signal receiver, more than one image frame of memory is typically needed for decoding an MPEG coded digital datastream, which represents I, P and B image frames as known. Three frames of memory are generally needed for decoding an MPEG datastream. Two frames of memory are needed to store reference I or P frame data, and an additional frame of memory is used to store B frame data.
An MPEG decoder includes a DPCM loop associated with a motion compensation function for producing finally decoded pixel samples, as known. As disclosed in copending U.S. patent application Ser. No. 08/579,192, the DPCM loop is advantageously modified by incorporating a data compression network. This compression network re-compresses decompressed MPEG data before being conveyed to a frame memory, thereby reducing the memory requirements of the MPEG decoder. The DPCM loop is arranged so that the value of a pixel to be compressed is dependent on the results of a predictor circuit evaluating pixels to the immediate left, directly above, and diagonally to the upper left of the pixel being processed. The predictor operation is a real-time, computationally intensive serial operation. The predictor operation is important since more than one pixel value is involved, and because good compression requires accurate prediction rather than a “guess” at a pixel value.